1. Field of the Invention
This invention relates to a photosensitive composition which is developable with an aqueous alkaline solution and is applicable particularly advantageously to the formation of a conductor circuit pattern, a barrier rib pattern, a dielectric pattern, a fluorescent pattern, and a black matrix on the front and the back substrate of a plasma display panel, and applicable also to the formation of an electric conductor, a resister, and a dielectric element for use in a fluorescent display tube and electronic parts. This invention also relates to such calcined patterns as a conductor pattern, a vitreous dielectric pattern, and a fluorescent pattern which are obtained by the use of the photosensitive composition.
2. Description of the Prior Art
A plasma display panel (hereinafter abbreviated as "PDP") is a planar display for exhibiting pictures and pieces of information by utilizing the light emitted by plasma discharge. It is classified under the DC type and the AC type according to the structure of panel and the mode of driving. The principle of color display by the PDP consists in generating plasma discharge in cells (discharge spaces) between two opposed electrodes severally formed on a front glass substrate and a back glass substrate separated by intervening ribs (barriers), and exciting the phosphor formed on the inner surface of the back glass substrate with the ultraviolet light generated by the discharge of such a gas as He or Xe sealed in the cells thereby inducing generation of visible lights of three primary colors. The cells in the DC type PDP are divided by the component ribs of a lattice, whereas those in the AC type PDP are divided by the ribs which are parallelly arranged on the face of the substrate. In either case, the cells are divided by ribs.
FIG. 1 illustrates a typical construction of the planar discharge type PDP using a three-electrode structure for full color display. On the lower face of a front glass substrate 1, many pairs of display electrodes 2a, 2b each comprising a transparent electrode 3a or 3b intended for discharge and a bus electrode 4a or 4b intended for lowering the line resistance of the transparent electrode are formed. On the display electrodes 2a, 2b, a transparent dielectric layer 5 (low melting glass) for accumulating electric charge is formed by printing and calcination. A protective layer (MgO) 6 is formed thereon by vacuum deposition. The protective layer 6 assumes the role of protecting the display electrodes and maintaining the state of discharge.
On a back glass substrate 7, ribs (barriers) 8 shaped like stripes and adapted to partition discharge spaces and address electrodes (data electrodes) 9 severally disposed in the discharge spaces are formed with prescribed pitches. On the inner faces of discharge spaces, fluorescent films of the three colors, i.e. red (10a), green (10b), and blue (10c), are laid out regularly. In the full color display, the fluorescent films of the three primary colors of red, green, and blue mentioned above jointly form one picture element.
The PDP described above is called a "planar discharge system" because an AC pulse voltage is applied between the pair of display electrodes 2a, 2b to induce discharge between the electrodes on one and the same substrate. It has a construction such that the ultraviolet light generated by discharge excites the fluorescent films 10a, 10b, and 10c of the back substrate 7 and the visible light consequently generated is seen through the transparent electrodes 3a, 3b of the front substrate 1 (reflection type).
Heretofore, the conductor patterns and the dielectric patterns on plasma display panels, fluorescent display tubes, and electronic parts have been formed by the screen printing method using a conductive paste or a glass paste which generally contains a very large amount of metallic powder or glass powder.
In recent years, however, the electronic parts have come to require the patterns formed thereon to have improved fineness. The formation of patterns by the conventional screen printing method has come to answer the demand for higher pattern fineness only with difficulty because it entails such problems as requiring skill on the part of workers, producing such blemishes as blurs or blots during the course of printing, and entailing deterioration of accuracy of registration of images due to elongation and contraction of the screen.
The photosensitive paste which contains a copolymer easily depolymerizable with heat in combination with an inorganic powder has come to find adoption as a means to fulfill the demand for improved pattern fineness. This photosensitive paste in most cases incorporates no photoreactive group in the base resin thereof and, therefore, is at a disadvantage in allowing photo-curing to proceed only to an insufficient depth during the course of exposure to light, producing inferior resolution after the development, and suffering the applied layer of paste to betray deficiency in fastness of adhesion to a substrate and in calcining properties. An effort to increase the depth of photo-curing and allow formation of patterns of improved fineness requires the photosensitive paste to incorporate additionally therein a reactive diluent (photopolymerizable monomer) in a large amount, with the awkward result that the proportion of the unsaturated group of the reactive diluent which remains in an unaltered form during the course of exposure to light will increase, the unaltered portion of the reactive diluent will be related during the course of calcining and consequently suffered to give rise to a calcining residue, and the relation will entail shrinkage and consequently compel the pattern to sustain distortion and contraction of line width and, in the worst case, result in breakage of the lines.
On the other hand, as a photosensitive, electroconductive paste which is intended for forming a conductor pattern on a ceramic circuit board, a paste composition containing an electroconductive powder, an acrylic copolymer possessing carboxyl groups and ethylenically unsaturated groups in the side chains thereof (for example, a polymer having glycidyl acrylates attached by addition reaction to a copolymer of methacrylic acid with methyl methacrylate and styrene), a photoreactive compound, and a photopolymerization initiator is disclosed in published Japanese Patent Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as "JP-A-") 5-67,405 and JP-A-5-271,576, for example.
The patent publications mentioned above teach that the photosensitive, electroconductive paste of the kind described above, when applied in a layer on a substrate and then subjected to exposure to light and development, is enabled to form a conductor circuit pattern and that the pattern nevertheless is fated to be calcined at a temperature in the range of 600.degree. to 1,000.degree. C. This photosensitive, electroconductive paste, therefore, is unfit for use in the PDP because it gives rise to a calcining residue at a temperature of not more than 600.degree. C. i.e. the calcining temperature which is used during the manufacture of PDP (since soda glass is generally used for the substrate of the PDP, the calcining must proceed at a temperature of not more than 600.degree. C.) and consequently induce deterioration of the electro-conductivity of the electrode circuits to be formed.
An attempt at simultaneous incorporation of low melting glass frit in the composition under discussion for the purpose of enabling the calcining to proceed at a relatively low temperature derives from the strong acidity of the methacrylic acid as a component of the copolymeric resin such problems as deteriorating the stability of storage of the produced composition, impairing the operational efficiency of coating work owing to gelation and degradation of flowing property, and disabling stable development with an aqueous alkaline solution.